image: (a) Energy transfer process of ideal thermochromic windows in winter daytime (left) and summer daytime (right). (b) The ideal spectrum of thermochromic windows against the normalized AM1.5 global solar spectrum (yellow shadow) and atmospheric transmissivity spectrum (blue shadow).
Credit: ©Science China Press
This study is led by Dr. Bin Zhu, Dr. Jia Zhu (College of Engineering and Applied Sciences, Nanjing University) and Dr. Qiuhong Zhang (School of Chemistry and Chemical Engineering, Nanjing University).
Windows, essential parts for occupants’ comfort, are considered among the least energy-efficient parts in buildings. Thermochromic smart windows are considered as an attractive energy-efficient auto-switching device due to their zero-energy-consumption characteristic, which can effectively reduce the energy consumption for building through passive light modulation including the transmissivity of visible (Vis, 0.38-0.78 μm), near-infrared (NIR, 0.78-2.5 μm), and the emissivity of mid-infrared (MIR, 2.5 -20 μm) in response to the ambient temperature change. However, developing thermochromic windows, that are capable of keeping high visible transmissivity with high near-infrared modulation and high mid-infrared modulation, is the “Holy Grail” and still remains a challenge.
Thus, Bin Zhu and Jia Zhu, together with Qiuhong Zhang, proposed a thermochromic smart window with high visible transmissivity more than 50% at both high and low temperatures and modulation on near-infrared transmissivity of 44.0% and mid-infrared emissivity of 76.5%, exhibiting excellent performance on thermal regulation and promoting the way to all-season energy-efficient buildings.
The team found an ideal thermochromic window should enable all the sunlight into the room for maximal solar harvesting in the daytime and possess low MIR emissivity to decrease the net radiative power outward under cold conditions such as winter daytime or frigid nighttime. Inversely, in hot conditions such as summer daytime or torrid daytime, the window should still present transparency in the visible band while simultaneously possessing high NIR reflectivity to reduce extra heat input and high MIR emissivity to improve radiative cooling performance.
The researchers designed a sandwiched structure consisting of a thermoplastic polyurethane (TPU) radiation layer with a one-dimensional photonic crystal, an adhesion tape layer, a two-way shape memory polymer (2W SMP) layer and an indium tin oxide (ITO) substrate, to enable the broadband regulation and reversible switching process. It can achieve a coiled state at low temperatures and possess the optical property with the visible transmissivity of 86.9%, the NIR transmissivity of 67.3% and the MIR emissivity of 12.3%. Inversely, it can achieve a completely flat state at high temperatures and possess the optical property with the visible transmissivity of 63.4%, the NIR transmissivity of 23.3% and the MIR emissivity of 88.7%. The reversibly switching performance agrees well with the proposed ideal spectrum of thermochromic windows.
Compared to traditional windows based on silica glass, this device shows 4℃ lower temperature in summer daytime, 2℃ higher in winter daytime, and 1℃ higher in spring nighttime in the field temperature tests. It is also verified that the device can spontaneously complete the whole transformation process in changing circumstances and ambient temperatures both in outdoor and indoor tests.
A medium office model in Energyplus software was also used to simulate the impacts on the monthly and annual energy consumption. It is calculated that when applying this designed thermochromic smart window as an alternative to commercial glass, the building will harvest more energy saving for each season, further achieving an average annual energy saving of 10-50 MJ m-2 across the world. It is expected that this designed device is compatible with conventional electrochromic smart windows which can regulate visible transparency actively and shows the potential to pave thermochromic smart windows for energy-efficient buildings towards global net-zero carbon.
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See the article:
A highly visible-transparent thermochromic smart window with broadband infrared modulation for all-season energy savings
https://doi.org/10.1093/nsr/nwae408
Journal
National Science Review